1. Field of the Invention
The present invention pertains to vapor recovery, and in particular, to the collection of condensable vapor from a solvent or a carrier gas.
2. Description of the Related Art
It is desirable to recapture vapors of volatile materials to reduce or eliminate their emission to the atmosphere, and in some circumstances, to recover the volatile materials for future use. Mechanical refrigeration systems are used to condense solvents from gaseous feed streams. For example, vapor recovery systems available from Edwards Engineering corporation of Pompton Plains, N.J. use multiple mechanical refrigeration stages to condense volatile materials in gaseous feed streams.
It is sometimes necessary to achieve low operating temperatures in order to improve vapor collection efficiencies as mandated by recent regulations. In order to achieve sufficiently low operating temperatures, multiple stages of mechanical refrigeration equipment must be employed, with the final stage associated with the lowest operating temperatures to which the gaseous feed stream, already refrigerated by preceding stages, is exposed. In one particular vapor recovery system, four mechanical stages are employed, and difficulties have been encountered in maintaining commercially attractive levels of operational reliability, particularly in the final refrigeration stage.
Techniques other than mechanical refrigeration have been used to recover condensable vapors from a gas stream. For example, U.S. Pat. No. 4,424,680 and British Patent 2,098,874 use direct contact techniques for removing condensable vapor content of a gas stream. U.S. Pat. No. 4,424,680 uses an inert gas such as nitrogen, while British Patent 2,098,874 uses either liquid argon gas or liquefied nitrogen gas. British Patent 838,222 directly contacts a gas stream with cooling water to remove vapor present in the gas stream.
Improvements in vapor recovery systems are constantly being sought. It has been observed that many mechanical refrigeration systems in use today, or proposed for use today, cannot meet recent stricter emission regulations due to inherent limitations of the refrigerants used. Accordingly, efficient, practical ways for recovering condensable vapors at lower temperatures are still being sought.
Also, a backup vapor recovery system has been sought for use during emergency conditions when mechanical failure of a mechanical refrigeration stage in an emission control system threatens shutdown or curtailment of an operating plant.